The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to release agent management (RAM) systems for a heat and pressure roll fuser.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions of the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an adhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E. I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate.
To obviate the foregoing toner offset problem it has been common practice to utilize toner release agents such as silicone oil, in particular, polydimethyl silicone oil, which is applied to the fuser roll surface to a thickness of the order of about 1 micron to act as a toner release material. These materials possess a relatively low surface energy and have been found to be materials that are suitable for use in the heated fuser roll environment. In practice, a thin layer of silicon oil is applied to the surface of the heated roll to form an interface between the roll surface and the toner image carried on the support material. Thus, a low surface energy, easily parted layer is presented to the toners that pass through the fuser nip and thereby prevents toner from adhering to the fuser roll surface. Apparatus for applying the release agent material to a fuser member is commonly referred to as a release agent management (RAM) system.
While the foregoing comments apply equally to color and black and white fusers color fusers require certain features not found in fusers used for fixing only black toner. A color fuser typically runs at a lower speed than a fuser that has to fuse only a black toner image. A color image which typically comprises three or four pile heights requires more heat addition to provide the desired gloss and fusion of the toner. This additional heat stresses the release performance of the fuser roll so it is generally required to add more oil to the roll surface during color copying compared to black only or monochrome copying. Prior art blade metering systems do not provide this extra oil at lower speeds. In fact, the hydrodynamic forces produce the opposite effect, i.e. less oil at lower speed.
It has been shown that oil film thickness passing under a metering blade (blade acting in a plow, scraping or doctor mode) is not strongly effected by the load on the blade at a load high enough to produce intimate metering roll-blade contact. As load is reduced film thickness increases slightly until suddenly areas of the blade "float" allowing a drastic film thickness increase while other areas are at their previous thin film. On the other hand blades mounted in a swiper mode do exhibit fairly large film thickness changes as a function of load but it is difficult to ever get as thin a film as a metering mode blade can produce.
Various systems have been used to deliver release agent fluid to the fuser roll including the use of oil soaked rolls and wicks with and without supply sumps as well as oil impregnated webs. Another type of RAM system is disclosed in U.S. Pat. No. 4,214,549 granted to Rabin Moser on Jul. 29, 1980. As disclosed therein, release agent material is contained in a sump from which it is dispensed using a metering roll and a donor roll, the former of which contacts the release agent material and the latter of which contacts the surface of the heated fuser roll.
The use of multiple or redundant components for effecting the same results in a xerographic apparatus is well known as illustrated in U.S. Pat. Nos. Re. 29,032 and 4,056,723. In the former, multiple blades are used for the removal of residual toner particles from the surface of an imaging surface. The purpose of the multiple blades is to extend the life of the cleaning system. In the latter patent, a plurality of corona discharge devices are disclosed. When one of the discharge devices becomes unusable another one is readily moved into operative position, the purpose being to prolong the life of the charging system.
Japanese Patent publication No. 1-189513 published on Mar. 7, 1991 discloses a blade contacting a toner conveyor roller. The roll is rotated in both the clockwise and counterclockwise directions. The blade contacts the roller in a wiping mode regardless of the direction of rotation of the toner conveying roller.
U.S. Pat. No. 3,940,282 granted to Stephen C. P. Hwa on Feb. 24, 1976 discloses wiping and doctoring blades for removing toner and debris from an imaging surface wherein the wiper blade deflects toner and debris removed from the imaging surface into a toner sump.
U.S. Pat. Nos. 4,264,191 and 4,279,500 granted on Oct. 2, 1974 and Jul. 21, 1981 to Bruce E. Thorpe and Gerbasi et al, respectively disclose a blade which is pressure engaged with an imaging member. In each of these patents the blade is used in both the wiping and doctoring modes but in order to accomplish this the imaging member is reverse rotated.
U.S. patent application No. 07/689,392 filed on Apr. 22, 1991 in the name of Siegl et al and assigned to the same assignee as the instant application discloses a dual action cleaning blade one side of which is provided with an abrasive coating material. The dual action feature stems from the fact that when the surface to be cleaned is moved in one direction the side of the blade containing the abrasive material abrades the surface with a wiping action. When the surface to be cleaned moves in the opposite direction the opposite side of the blade does the wiping and thus the cleaning.